The invention relates to the field of optical fibers, and more precisely to the fabrication of optical fibers by depositing a buildup material on a preform by means of a plasma torch.
Plasma buildup of a “primary” optical fiber preform is a technique that serves to increase the diameter of the preform considerably, and consequently to enable optical fibers of greater length to be drawn. This technique is described in particular in documents EP 0 401 742, EP 0 450 465, and EP 0 658 520.
Because of the high temperatures generated by the plasma produced by the plasma torch, interaction between the nitrogen and the oxygen contained in the confined enclosure where buildup is performed generates nitrogen oxides (NOx).
These nitrogen oxides are exhausted from the enclosure and collected by a processing installation for restricting rejection to the atmosphere in compliance with antipollution standards. Such processing installations generally make use of a depollution method referred to as selective catalytic reduction (SCR) which consists in heating the residual gases to a temperature lying in the range 300° C. to 400° C. and in making them react with a reducing gas in the presence of a catalyst. Such an installation is expensive both in terms of its own manufacture and installation, and in terms of its running costs. In addition, that type of installation is relatively bulky. Furthermore, heating the residual gases by means of natural gas gives rise to a large discharge of carbon dioxide gas. Finally, the residual gases for processing generally contain very fine particles of silica, typically of submicron size, and these can lead to premature wear of the catalyst, and consequently to a significant extra amount of operating cost.
An object of the invention is thus to improve the situation.